Undesired heat loss from production tubing as well as uncontrolled heat transfer to outer annuli can be detrimental to the mechanical integrity of outer annuli, cause productivity losses from the well due to deposition of paraffin and asphaltene materials, accelerate the formation of gas hydrates, and destabilize the permafrost in arctic type regions.
Early methods into controlling heat loss and enhancing oil recovery were focused on steam injection operations. For applications where the packer annulus was gas-filled, wellbore heat losses from refluxing annuli were found to be three to six times higher than anticipated for insulated tubing and only 30 to 40 percent less had the injection tubing not have been insulated.
Silicate foams were among the first insulating packer fluids. Such foams were employed in steam injection applications wherein a solution of sodium silicate was placed in a packed-off annulus, and then steam was injected down the tubing. The hot tubing caused the silicate solution to boil, leaving a coating of insulating material, silicate foam of ¼ to ½ inch thick, on the hot tubing surf ace. Silicate solution that remained in the annulus after steaming for several hours was removed from the annulus by displacing it with water which was removed by gas-lifting or swabbing. The foam insulator exhibited thermal conductivity of about 0.017 Btu/(hr·ft·° F.). However, difficulties were encountered in boiling off the solutions to form the foam. “Hot spots” were also observed to develop adjacent to the uninsulated couplings.
To prevent thermal refluxing, an insulating fluid that filled the entire annulus was chosen as an alternative to the gas filled annulus. Such fluids avoided unwanted heat loss as a result of reduced thermal conduction and/or convection. Oils, such as gelatinous oil based fluids exhibited relatively low thermal conductivity (0.08 Btu/(hr·ft·° F.). For instance, the relative thermal conductivity of this type of fluid was approximately 13 percent that of water. However, environmental restrictions limited the application of such oils. Furthermore, the long-term incompatibility with various elastomers presented concerns.
As an alternative to chemical methods, vacuum insulated tubing was proposed to solve the problem of paraffin deposition in the production tubing. While insulated tubing proved to be an effective method for wellbore insulation, actual heat losses were significant. Heat loss through couplings and other internal structures such as centralizers and valves were seen to account for up to 50 percent of the total heat loss. To fully achieve the potential of insulated tubing, selected rubber-insulated couplings were tested along with a thermal pipe coating. Although improved thermal performance was obtained, maintaining the annulus dry over a long period was difficult, and, heat loss through refluxing could still occur because of damaged and scraped coating, and downhole centralizers, valves and gauges. Furthermore, the cost of vacuum insulated tubing can be prohibitive for many projects.
It was found that such problems could be controlled effectively by the use of specially designed aqueous-based (oil-free) insulating packer fluids. Such fluids were found to secure the insulation of the wellbore and to reduce the amount of heat transfer from the production tubing to the surrounding wellbore, internal annuli, and riser. Conventional packer fluids, such as clear brines under natural convection, were found to transfer heat by a factor of 10 to 20 over molecular conduction. Free convection is fluid motion caused by the variation of fluid density with temperature. Increasing fluid viscosity decreases fluid motion, and correspondingly, decreases free annular convection.
Thus, the desired rheological profile for an insulating fluid began to include high viscosity properties at low shear rate in order to reduce the free fluid convection caused by temperature differential. Additionally, a low viscosity at high shear rate is desired to facilitate the placement of the insulating fluid at the desired location. Specific rheological properties were selected based on the specific well application.
Exemplary of such aqueous based insulating fluids are the non-crosslinked insulating fluids disclosed in U.S. Pat. No. 6,489,270, herein incorporated by reference. Such aqueous based insulating fluids proved to be solids-free, non-damaging, environmentally friendly, and highly insulating. Their fluid viscosity made it easy to blend and pump them into the annulus; their fluid density being controlled by the amount and type of dissolved salt needed to provide positive control of the wellbore pressure without the risk of solid settling and separation.
Such fluids, when added either into an annulus or riser, effectively reduced undesired heat loss from the production tubing, or heat transfer to outer annuli. In some cases, heat loss from the produced fluids due to conduction and convection can be reduced by more than 90% when compared with conventional packer fluids. Fluids having improved insulation properties have been sought.